Strongly localized polaritons in an array of trapped two-level atoms interacting with a light field

We propose a new type of spatially periodic structure, i.e. polaritonic crystal (PolC), to observe a 'slow'/'stopped' light phenomenon due to coupled atom-field states (polaritons) in a lattice. Under the tight-binding approximation, such a system realizes an array of weakly coupled trapped two-component atomic ensembles interacting with the optical field in a tunnel-coupled one-dimensional cavity array. We have shown that the phase transition to the superfluid Bardeen-Cooper-Schrieffer state, a so-called (BCS)-type state of low branch polaritons, occurs under the strong coupling condition. Such a transition results in the appearance of a macroscopic polarization of the atomic medium at non-zero frequency. The principal result is that the group velocity of polaritons depends essentially on the order parameter of the system, i.e. on the average photon number in the cavity array.